Ion mobility detectors are the primary instruments used in the field of plasma chromatography. Generally, the operation of an ion mobility detector is similar to the operation of a time of flight mass spectrometer, the obvious difference being that a time of flight mass spectrometer operates in a vacuum where the mean free path of the contained gases is many times the dimension of the gas container, while the ion mobility detector operates generally at atomspheric pressure where the mean free path of the contained gases is a small fraction of the dimensions of the container. More particularly, a typical ion mobility detector is comprised of a ionization source, an ion reactant region, an ion drift region and an ion injection shutter or grid interposed between the ion reactant region and the ion drift region. A carrier gas, normally purified atmospheric air, is introduced into the ion mobility detector with a gaseous sample of a material, whose identity is to be characterized by the ion mobility detector, so that the gaseous mixture is exposed to the ionization source. As a result, portions of both the carrier gas and the sample are directly ionized by the ionization source. However, as known to those practicing in this art, the characteristics of the carrier gas and the sample are usually such that the molecules of the carrier gas are more easily directly ionized by the ionization source than are the molecules of the sample. At this time the gaseous mixture is contained within the reactant region. Since the mean free path is many times smaller than the dimensions of the reactant region there are multiple collisions between the molecules of the carrier and sample gases. As also known to those skilled in the art, the tendency of these collisions is to transfer the ion charge from the carrier molecules to the sample molecules, thereby ionizing the sample gas mainly by this secondary ionization process.
The charged particles or ions, now mainly derived from the sample, are accelerated to a terminal velocity under the influence of a field potential gradient within the reactant region toward an ion injection grid which, as mentioned earlier, separates the reactant region from the drift region. The grid is normally electrically charged to prevent the transfer of ions from the reactant region to the drift region. Periodically, the grid is deenergized for a short time period to permit a pulse of ions to pass therethrough into the drift region. Here, the ions, under the influence of an electrostatic drift field are accelerated to an electrometer detector which terminates the drift region. The time of arrival of each ion at the electrometer detector, relative to the time the grid was opened, is determined by the ion's mobility through the non-ionized gas occupying the drift region. The heavier ions characteristically move more slowly through the drift region and arrive at the electrometer detector after longer drift times than lighter ions. It is thus possible to characterize the ions and hence, the sample by observing the time between the opening of the grid and the arrival at the electrometer detector.
In a practical sense, an ion mobility detector may be used to determine whether a certain sample is present in an environment, such as a certain contaminant in atmospheric air. In this case the electrometer detector is sampled at predetermined times after the grid is opened to discover whether pulses of ions are then arriving at the electrometer detector. If the proper combination of responses is obtained then it can be concluded that the contaminant is present.
In the prior art, the reactant region and the drift region are normally defined by the interior surfaces of tubular structures which are constructed of alternating electrically conductive and non-conducting rings. In the art, the conductive rings are termed guard rings. The region fields, above termed the field potential gradient in the case of the reactant region field and the electrostatic drift field in the case of the drift region, are generated by connecting adjacent guard rings through resistors and connecting the end rings respectively to the terminals of a voltage source. There thus results a series of conductive rings with ascending voltage levels impressed thereon so that the conductive rings, as interleaved with the non-conductive rings, comprise a tube having a longitudinal axis which coincides with the longitudinal axis of the above mentioned electrostatic field. It is believed in the prior art that the guard ring structure is needed to ensure an easily cleanable unit which is not apt to adsorb or absorb extraneous molecules which could cause erroneous responses.